The present invention relates to an integrated circuit for television receivers comprising a sync separator with a clamping circuit which makes the relative separation level for the synchronizing signal independent of the amplitude of the latter. The separation level and the porch are integrated by means of a first and a second RC section of the kind set forth in the preamble of the claim. The integrated circuit further includes a coincidence stage, the voltage-controlled horizontal oscillator, the phase comparator for the horizontal oscillator and horizontal synchronizing signals, a gate circuit, a vertical-synchronizing-pulse integrator, and the vertical oscillator; for further details of these subcircuits, see the preamble of the claim. An integrated circuit of this kind is known under the designations TDA 1950 and TDA 1950 F described in the 1981 data book "Integrierte Schaltungen fur Rundfunk- und Fernsehempfanger", ITT-Intermetall, Freiburg, Edition 1981/6, pages 100 to 103. Further details of the coincidence stage and the gate circuit are also given in the journal "Elektronik aktuell", 1976, No. 2, pages 7 to 14, in connection with the integrated circuits TDA 9400 and TDA 9500. Both publications originate from the Applicants.
In the case of the prior art clamping circuit with two series RC sections and an interconnecting resistor, the switching level is the same for all horizontal synchronizing pulses because the charging current and the discharge current of the capacitor C1 are in equilibrium. The time constant of this RC section is chosen so that during amplitude variations of the composite color signal or during DC level variations caused e.g., by power-line hum in the composite color signal, the separation level remains constant in relation to the peak value. This time constant is generally shorter than 20 ms.
However, this circuit has the disadvantage that during the vertical synchronizing pulses contained in the composite color signal, the separation level shifts in such a way that the separation level for the subsequent horizontal synchronizing pulses of the next line is greater than that in the normal case described. Because of the finite rise time of the horizontal synchronizing pulses, the separated horizontal synchronizing pulses appearing at the output of the sync separator are slightly shifted in phase, so that at the beginning of each picture, the horizontal synchronization shows a slight phase error.
Another disadvantage of the prior art circuit lies in the fact that the separation level may be shifted by wide interference pulses to the point that the subsequent horizontal synchronizing pulses cannot be separated for a certain time.